Method of preparing catalyst for manufacturing carbon nanotubes

ABSTRACT

A novel method of forming catalyst particles, on which carbon nanotubes grow based, on a substrate with increased uniformity, and a method of synthesizing carbon nanotubes having improved uniformity are provided. A catalytic metal precursor solution is applied to a substrate. The applied catalytic metal precursor solution is freeze-dried, and then reduced to catalytic metal. The method of forming catalyst particles can minimize agglomeration and/or recrystallization of catalyst particles when forming the catalyst particles by freeze-drying the catalyst metal precursor solution. The catalyst particles formed by the method has a very uniform particle size and are very uniformly distributed on the substrate.

CLAIM OF PRIORITY

This application claims the priority of Korean Patent Application No. 10-2004-0046552, filed on Jun. 22, 2004, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of preparing a catalyst for manufacturing carbon nanotubes and a method of manufacturing carbon nanotubes using the same.

2. Description of the Related Art

A carbon nanotube has a cylindrical structure having a diameter of several nano-meter and a very large aspect ratio of about 10 to 1,000. In the carbon nanotube, carbon atoms are generally arranged in a hexagonal honeycomb pattern. One carbon atom bonds to three adjacent carbon atoms. The carbon nanotube may be a conductor or a semiconductor according to its structure. As a conductor, the carbon nanotube has high electroconductivity. Also, the carbon nanotube has superior mechanical strength, Young's modulus of tera level, and high heat conductivity. The carbon nanotube having these properties can be advantageously used in various technical fields, such as an emitter of FED, a cathode material for a secondary battery, a catalyst support of a fuel cell, a high strength composite, and the like.

Examples of a method of preparing the carbon nanotube include arc discharge, laser deposition, plasma enhanced chemical vapor deposition (PECVD), chemical vapor deposition, vapor phase growth, electrolysis, and the like. The vapor phase growth is suitable for preparing the carbon nanotube in bulk form since it synthesizes the carbon nanotube in a vapor phase by directly supplying a reaction gas and a catalytic metal into a reactor without using a substrate. The arc discharge and the laser deposition have relatively low yields of carbon nanotubes. When using the arc discharge and the laser deposition, It is difficult to control the diameter and the length of the carbon nanotube. Further, in the arc discharge and the laser deposition, clusters of amorphous carbon besides the carbon nanotubes are produced in a large amount, and thus a complicated purifying process must be performed.

Chemical vapor deposition (CVD) methods, such as thermal chemical vapor deposition, low pressure chemical vapor deposition and PECVD are generally used to form carbon nanotubes on a substrate. In the PECVD, the carbon nanotubes can be synthesized at low temperatures by activating gas with a plasma. In the PECVD, it is relatively easy to control the diameter, the length, the density, etc. of the carbon nanotube.

In the case of chemical vapor deposition (CVD) methods, catalyst particles, on which carbon nanotubes grow based, are first dispersed on a substrate in order to obtain a uniform density of the carbon nanotubes formed on the substrate.

For example, Korean Patent Laid-Open Publication No. 2001-0049398 discloses a method of forming a plurality of catalyst particles by forming a catalytic metal film on a substrate and etching the catalytic metal film with an etching gas.

In addition, Chemical Physics Letter, vol. 377 p. 49, 2003 discloses a method of forming catalyst particles on a substrate by applying a catalytic metal precursor solution to the substrate, and then drying and heat-treating the applied catalytic metal precursor solution. However, in this case, recrystallization and agglomeration of the catalytic metals occur during the drying and heat-treatment processes so that uniformity of the catalyst particles formed on the substrate is deteriorated. Due to the deterioration in the uniformity of the catalyst particles formed on the substrate, the uniformity of the diameter and production density of the carbon nanotubes grown on the basis of the catalytic particles are both deteriorated.

The uniformity of the catalyst particles formed on the substrate can be evaluated by measuring the uniformity of the particle sizes of the catalyst particles and the uniformity of the production density of the catalyst particles. The uniformity of the catalyst particles formed by the conventional methods is not sufficient. Thus, a novel method of forming catalyst particles in order to improve the uniformity of the catalyst particles formed on a substrate is needed.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide a novel method of forming catalyst particles.

It is a further object of the present invention to provide a novel method of forming catalyst particles with increased uniformity, on which carbon nanotubes grow based, on a substrate.

It is also an object of the present invention to provide a method of synthesizing carbon nanotubes with improved uniformity.

According to an aspect of the present invention, there is provided a method of forming catalyst particles, the method including: applying a catalytic metal precursor solution to a substrate, the catalytic metal precursor solution comprising a catalytic metal precursor and a solvent; freeze-drying the catalytic metal precursor solution applied to the substrate; and reducing the freeze-dried catalytic metal precursor to a catalytic metal.

It is preferred that the catalytic metal precursor solution is freeze-dried by cooling the catalytic metal precursor solution applied to the substrate below the freezing point of the catalytic metal precursor solution and evaporating the solvent in the catalytic metal precursor solution under a reduced pressure.

The method of forming catalyst particles can minimize the agglomeration and/or recrystallization of the catalytic metal particles when forming the catalytic metal particles by freeze-drying the catalytic metal precursor solution. Therefore, the catalytic metal particles formed by the method of the present invention have particle sizes with very high uniformity and are very uniformly distributed on the substrate.

According to another aspect of the present invention, there is provided a method of manufacturing carbon nanotubes, the method including: forming catalyst particles, on which carbon nanotubes grow based, on a substrate by applying a catalytic metal precursor solution to the substrate, freeze-drying the catalytic metal precursor solution applied to the substrate, and reducing the freeze-dried catalytic metal precursor to a catalytic metal; and growing carbon nanotubes on the catalyst particles by supplying a carbon source to the catalyst particles.

According to another aspect of the present invention, there is provided a method of manufacturing carbon nanotubes, the method including: applying a catalytic metal precursor solution to the substrate; cooling the catalytic metal precursor solution applied to the substrate below the freezing point of the catalytic metal precursor solution; evaporating the solvent in the catalytic metal precursor solution under a reduced pressure to form catalytic metal precursor particles; converting the catalytic metal precursor particles to a catalyst particles; and growing carbon nanotubes on the catalyst particles.

It is preferred that the catalytic metal precursor particles is converted into the catalyst particles by heating the catalytic metal precursor particles in an oxidation atmosphere to oxidize the catalytic metal precursor particles and reducing the oxidized catalytic metal precursor particles to the catalyst particles by heat-treatment or plasma-treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the present invention, and many of the above and other features and advantages of the present invention, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:

FIG. 1 is an optical microscopic image illustrating catalyst particles for manufacturing carbon nanotubes, prepared according to an Example of the present invention;

FIG. 2 is an electron microscopic image illustrating a side view of carbon nanotubes prepared according to an Example of the present invention;

FIG. 3 is an electron microscopic image illustrating a top view of carbon nanotubes prepared according to an Example of the present invention;

FIG. 4 is an optical microscopic image illustrating catalyst particles for manufacturing carbon nanotubes, prepared according to a Comparative Example;

FIG. 5 is an enlarged view of a part of FIG. 4; and

FIG. 6 is an electron microscopic image illustrating a state of carbon nanotubes prepared according to a Comparative Example.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a method of forming catalyst particles, on which carbon nanotubes grow based, on a substrate according to an embodiment of the present invention will be described in detail.

The method of forming catalyst particles includes: applying a catalytic metal precursor solution to a substrate; freeze-drying the catalytic metal precursor solution applied to the substrate; and reducing the freeze-dried catalytic metal precursor to catalytic metal.

The catalytic metal precursor solution includes a catalytic metal precursor and a solvent for dissolving the catalytic metal precursor.

The catalytic metal precursor may be any material that can be converted to metal particles, on which carbon nanotubes can grow based. An example of the catalytic metal precursor includes an organo-metallic compound. The organo-metallic compound can contain at least one metal element selected from the group consisting of Fe, Co, Ni, Y, Mo, Cu, Pt, V, and Ti. Examples of the organo-metallic compound include iron acetate, iron oxalate, cobalt acetate, nickel acetate, ferrocene, or a mixture thereof.

The solvent may be any liquid material that can dissolve the catalytic metal precursor. Examples of the solvent include ethanol, ethylene glycol, polyethylene glycol, polyvinyl alcohol, and a mixture thereof.

The concentration of the catalytic metal precursor in the catalytic metal precursor solution is not particularly limited. If the concentration of the catalytic metal precursor in the catalytic metal precursor solution is too low, the carbon nanotubes may not be generated in a subsequent process of manufacturing. If the concentration of the catalytic metal precursor in the catalytic metal precursor solution is too high, the diameter of the carbon nanotubes generated in a subsequent process of manufacturing may be increased or the crystallinity of the carbon nanotubes generated or carbon nanofibers may be reduced. The concentration of the catalytic metal precursor in the catalytic metal precursor solution can preferably be about 10 mM to 200 mM.

The substrate may be composed of any material, to which catalyst particles can be attached, for example, a metal with a high melting point, such as Mo, Cr, and W, silicon, glass, plastic, quartz, and the like.

The method of applying the catalytic metal precursor solution to the substrate may be any method capable of uniformly coating the solution on the surface of the substrate. Examples of the method include dip coating, evaporation coating, screen printing, spin coating, and the like. These methods can also be used in a combination.

The catalytic metal precursor solution can be applied to the entire surface of the substrate or on only a part of the surface of the substrate.

The catalytic metal precursor solution applied to the substrate is freeze-dried. The freeze-drying process includes cooling the catalytic metal precursor solution applied to the substrate below the freezing point of the catalytic metal precursor solution and evaporating the solvent in the catalytic metal precursor solution under a reduced pressure.

The freezing point of the catalytic metal precursor solution may vary depending on a composition of the catalytic metal precursor solution. That is, the freezing point of the catalytic metal precursor solution can be determined by the type of the catalytic metal precursor, the type of the solvent, the concentration of the catalytic metal precursor, and the like. The freezing point of the catalytic metal precursor solution can be easily determined by those skilled in the art through thermodynamical calculation and the method of trial and error. The freezing point of the catalytic metal precursor solution can also be selected by adjusting the composition of the catalytic metal precursor solution.

The process of cooling the catalytic metal precursor solution applied to the substrate below the freezing point of the catalyst solution can be performed using a cooling method suitable for the freezing point of the catalytic metal precursor solution. For example, a freezer, liquid nitrogen, etc. can be used. When using liquid nitrogen, the catalytic metal precursor solution applied to the substrate can be cooled to below the freezing point of the catalytic metal precursor solution by dipping the substrate with the catalytic metal precursor solution applied thereto in liquid nitrogen.

After the catalytic metal precursor solution applied to the substrate freezes, the substrate is subjected to a reduced pressure in order to allow the solvent in the frozen catalytic metal precursor solution to evaporate. For example, the substrate with the frozen catalytic metal precursor solution applied thereto is placed in a vacuum chamber, and then the pressure of the inside of the vacuum chamber is reduced.

The reduced pressure should be sufficient to allow the solvent in the frozen catalytic metal precursor solution to evaporate. Hereinafter, the reduced pressure sufficient to allow the solvent in the frozen catalytic metal precursor solution to evaporate is abbreviated to “evaporation pressure”. The evaporation pressure can vary depending on a composition the used catalytic metal precursor solution. That is, the evaporation pressure can be determined by the type of the catalytic metal precursor, the type of the solvent, the concentration of the catalytic metal precursor, freezing point, and the like. The evaporation pressure of the catalytic metal precursor solution can be easily determined by those skilled in the art through thermodynamical calculation and the method of trial and error. The evaporation pressure of the catalytic metal precursor solution can also be selected by adjusting the composition of the catalytic metal precursor solution, freezing point, and the like.

The solvent in the frozen catalytic metal precursor solution is removed through the evaporation. As a result, catalytic metal precursor components are formed in a particle form on the substrate. It is noted that the catalytic metal precursor particles formed according to the present method have particle size with high uniformity and a uniform distribution on the substrate.

Subsequently, the catalytic metal precursor particles formed on the substrate are reduced to catalytic metal particles. The process of reducing the catalytic metal precursor particles to catalytic metal particles can be performed, for example, as follows. First, the catalytic metal precursor is converted into an oxide through heat-treatment in an oxidation atmosphere, and then the oxide is heat-treated or plasma treated in a reduction atmosphere to be reduced to a metal. The process of reducing the catalytic metal precursor can be performed by various methods known in the art, and thus, the detailed description thereof will be omitted herein.

FIG. 1 is an electron microscopic image of catalytic metal particles prepared according to an Example of the present invention. Referring to FIG. 1, the catalytic metal particles are uniformly distributed on the substrate and the particle sizes thereof are relatively uniform.

A method of manufacturing carbon nanotubes according to an embodiment of the present invention will now be described in more detail.

The method of manufacturing carbon nanotubes includes forming catalyst particles, on which carbon nanotubes grow based, on a substrate by applying a catalytic metal precursor solution to the substrate, freeze-drying the catalytic metal precursor solution applied to the substrate, and reducing the freeze-dried catalytic metal precursor to catalytic metal; and growing carbon nanotubes on the catalyst particles by supplying a carbon source to the catalyst particles.

The process of forming catalyst particles on the substrate is performed in the same manner as described in the method of forming catalyst particles.

The process of growing carbon nanotubes on the catalyst particles by supplying the carbon source to the catalyst particles may be performed by various methods useful for the manufacture of carbon nanotubes.

For example, the process of growing carbon nanotubes on the catalyst particles includes placing the substrate on which the catalyst particles are formed in a reaction chamber, supplying carbon precursor gas into the reaction chamber, and decomposing the carbon precursor gas in the reaction chamber to supply carbon to the catalyst particles.

The process of growing the carbon nanotubes can be performed by low pressure chemical vapor deposition, thermal chemical vapor deposition, PECVD, or a combination thereof.

Examples of the carbon precursor gas include carbon containing compounds such as acetylene, methane, propane, ethylene, carbon monoxide, carbon dioxide, alcohol, and benzene.

If the internal temperature of the reaction chamber is too low, the crystallinity of the generated carbon nanotubes may be diminished. If the internal temperature of the reaction chamber is too high, the carbon nanotubes may not be formed. In view of this, the internal temperature of the reaction chamber may preferably be in the range of about 450 to 1100° C.

Other conditions in the process of growing carbon nanotubes may typically be those suitable for the growth of carbon nanotubes and be easily selected by those skilled in the art according to specific application purposes. Thus, detailed description of other conditions will be omitted herein.

Since in the method of manufacturing carbon nanotubes of the present embodiment, carbon nanotubes grow based on catalyst particles that have a uniform particle size and are uniformly distributed on the substrate, the uniformity of the resulting carbon nanotubes is also highly improved. The uniformity of carbon nanotubes is evaluated by the uniformity of the lengths and diameters of the carbon nanotubes. The lengths and diameters of carbon nanotubes can be measured by an electron microscope and a transmittance electron microscope, respectively.

Further, the vertical orientation characteristic of carbon nanotubes manufactured by the method of the present embodiment is very good. This can be confirmed from an electron microscopic image of FIG. 2. FIG. 2 is an image showing a side view of carbon nanotubes prepared in an Example of the present invention. Referring to FIG. 2, the carbon nanotubes prepared according to the method of the present embodiment are vertically oriented without being entangled with one another.

FIG. 3 is an image showing a top view of carbon nanotubes prepared in an Example of the present invention. Referring to FIG. 3, the production density of carbon nanotubes prepared according to the method of the present embodiment is uniform.

EXAMPLE

A 40 mM iron acetate solution was prepared using ethanol and ethylene glycol as a solvent. 20 mL of ethanol and 20 mL of ethylene glycol were added to 0.1 g of iron acetate powder to obtain a solution having a proper viscosity. A silicon substrate with a diameter of 20.32 cm was dipped in the obtained solution. The coated substrate was cooled immediately with liquid nitrogen and then transferred into a vacuum chamber. Then, a vacuum less than 0.1 mmHg was applied to the chamber in order to evaporate the solvent. To minimize an amount of the remained solvent, the substrate was further heated at 100° C.

The freeze-dried substrate was heat-treated at 300° C. for 10 minutes in order to oxidize the iron acetate. Then, the substrate was subjected to a reduction treatment with hydrogen at 600° C.

As a result, the iron particles were uniformly formed on the substrate. FIG. 1 is an electron microscopic image of iron particles formed on the silicon substrate according to the present Example. Referring to FIG. 1, the iron particles are uniformly distributed on the substrate and the particle sizes thereof are relatively uniform.

The substrate having iron particles formed thereon was placed in a reaction chamber for chemical vapor deposition (CVD), an internal temperature of which is 600° C., and then a mixed gas of carbon monoxide and hydrogen (weight ratio 1:2) was supplied to the reaction chamber for 20 minutes to synthesize carbon nanotubes based on the iron particles.

FIG. 2 is an image showing a side view of carbon nanotubes prepared in the present Example. As is apparent from FIG. 2, the carbon nanotubes prepared in the present Example are vertically oriented without being entangled with one another. FIG. 3 is an image showing a top view of carbon nanotubes prepared in the present Example. It can be seen from FIG. 3 that the production density of carbon nanotubes prepared in the present Example is uniform.

To evaluate the uniformity of the formed carbon nanotubes, the measurements of the lengths and the diameters of the carbon nanotubes using an electron microscope and a transmittance electron microscope, respectively, were performed on the respective parts of the substrate, which were equally divided into 9 parts. As a result, it is confirmed that carbon nanotubes on the substrate equally divided into 9 parts has a uniformity within ±5%.

Comparitive Example

Carbon nanotubes were synthesized in the same manner as in the above Example, except that the iron acetate solution applied to the substrate was not freeze-dried but naturally dried.

FIG. 4 is an optical microscopic image showing iron particles prepared in Comparative Example. FIG. 5 is an enlarged view of a part of FIG. 4. As seen from FIGS. 4 and 5, iron particles prepared in Comparative Example have no uniformity.

FIG. 6 is an electron microscopic image showing a form of carbon nanotube clusters synthesized in Comparative Example. Referring to FIG. 6, the carbon nanotubes synthesized in Comparative Example are partially lumped on the substrate, are entangled with one another, and not vertically oriented.

The method of forming catalyst particles according to an embodiment of the present invention can minimize the agglomeration and/or recrystallization of the catalyst particles when forming the catalyst particles by freeze-drying the catalyst metal precursor solution. The catalyst particles formed by the method of the present embodiment have a very uniform particle size and a very uniform distribution on the substrate.

In the method of manufacturing carbon nanotubes according to another embodiment of the present invention, the carbon nanotubes grow based on the catalyst particles having a uniform particle size and a uniform distribution on the substrate, and thus, the synthesized carbon nanotubes have highly improved uniformity.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the following claims. 

1. A method of preparing catalyst particles for carbon nanotube manufacture, the method comprising: applying a catalytic metal precursor solution to a substrate, the catalytic metal precursor solution comprising a catalytic metal precursor and a solvent; freeze-drying the catalytic metal precursor solution applied to the substrate; and reducing the freeze-dried catalytic metal precursor to catalytic metal.
 2. The method of claim 1, wherein the catalytic metal precursor is an organo-metallic compound.
 3. The method of claim 2, wherein the catalytic metal precursor is an organo-metallic compound containing at least one metal element selected from the group consisting of Fe, Co, Ni, Y, Mo, Cu, Pt, V, and Ti.
 4. The method of claim 1, wherein the solvent of the catalytic metal precursor solution is ethanol, ethylene glycol, polyethylene glycol, polyvinyl alcohol, or a mixture thereof.
 5. The method of claim 1, wherein the concentration of the catalytic metal precursor in the catalytic metal precursor solution is 10 mM to 200 mM.
 6. The method of claim 1, wherein the step of freeze-drying the catalytic metal precursor solution comprises cooling the catalytic metal precursor solution applied to the substrate below the freezing point of the catalytic metal precursor solution and evaporating the solvent in the catalytic metal precursor solution under a reduced pressure.
 7. The method of claim 6, wherein the step of cooling the catalytic metal precursor solution comprises using a freezer or liquid nitrogen.
 8. Catalyst particles prepared by the method of claim
 1. 9. A method of manufacturing carbon nanotubes, comprising utilizing the catalyst particles of claim
 8. 10. A method of manufacturing carbon nanotubes, the method comprising: forming catalyst particles on a substrate by applying a catalytic metal precursor solution comprising a catalytic metal precursor and a solvent to the substrate, freeze-drying the catalytic metal precursor solution applied to the substrate, and reducing the freeze-dried catalytic metal precursor to a catalytic metal; and growing carbon nanotubes on the catalyst particles by supplying a carbon source to the catalyst particles.
 11. The method of claim 10, wherein the catalytic metal precursor is an organo-metallic compound, and the solvent is ethanol, ethylene glycol, polyethylene glycol, polyvinyl alcohol, or a mixture thereof.
 12. The method of claim 10, wherein the catalytic metal precursor is an organo-metallic compound containing at least one metal element selected from the group consisting of Fe, Co, Ni, Y, Mo, Cu, Pt, V, and Ti.
 13. The method of claim 10, wherein the concentration of the catalytic metal precursor in the catalytic metal precursor solution is 10 mM to 200 mM.
 14. The method of claim 10, wherein the step of freeze-drying the catalytic metal precursor solution comprises cooling the catalytic metal precursor solution applied to the substrate below the freezing point of the catalytic metal precursor solution and evaporating the solvent in the catalytic metal precursor solution under a reduced pressure.
 15. The method of claim 10, wherein the step of cooling the catalytic metal precursor solution comprises using a freezer or liquid nitrogen.
 16. The method of 10, wherein the step of growing the carbon nanotubes comprises placing the substrate on which the catalyst particles are formed in a reaction chamber, supplying carbon precursor gas into the reaction chamber, and decomposing the carbon precursor gas in the reaction chamber to supply carbon to the catalyst particles.
 17. The method of 16, wherein the internal temperature of the reaction chamber is in the range of about 450 to 1100° C.
 18. The method of claim 10, wherein the reduction of the freeze-dried catalytic metal precursor to a catalytic metal comprises heating the freeze-dried catalytic metal precursor in an oxidation atmosphere to oxidize the catalytic metal precursor, and reducing the oxidized catalytic metal precursor to the catalyst metal by heat-treatment or plasma-treatment.
 19. A method of manufacturing carbon nanotubes, the method comprising: applying a catalytic metal precursor solution to the substrate, the catalytic metal precursor solution comprising a catalytic metal precursor dissolved in a solvent; cooling the catalytic metal precursor solution applied to the substrate below the freezing point of the catalytic metal precursor solution; evaporating the solvent in the catalytic metal precursor solution to form catalytic metal precursor particles; converting the catalytic metal precursor particles to a catalyst particles; and growing carbon nanotubes on the catalyst particles.
 20. The method of claim 19, wherein the step of converting the catalytic metal precursor particles to the catalyst particles comprises heating the catalytic metal precursor particles in an oxidation atmosphere to oxidize the catalytic metal precursor particles, and reducing the oxidized catalytic metal precursor particles to the catalyst particles by heat-treatment or plasma-treatment. 